There is no official definition for severe service. It may be taken to mean operating conditions in which a valve replacement is costly or lowers the process capacity.
There is a global need to reduce the process production cost to enhance profitability in all sectors that involve severe service conditions. These range from oil & gas and petrochemicals to nuclear and power generation, mineral processing and mining.
Designers and engineers are making efforts to achieve this through different means. The most appropriate way is to increase uptime and efficiency by effectively controlling the process parameters like effective shut-off and optimized flow control.
Safety optimization also plays a vital role, as reducing change-outs leads to a safer production environment. In addition, companies are trying to minimize inventory of devices, including pumps and valves, and the required handling. At the same time, facility owners are expecting huge turn over from their assets. Hence, increased process capacities are leading to fewer (but larger diameter) pipes and devices and less instrumentation for the same product flow.
This shows that individual system components, besides having to be larger for use with broader pipe diameters, need to tolerate long-time exposure to harsh environments so as to reduce the requirement for in-service maintenance and replacement.
Components, including valves and valve balls need to be strong to suit the desired application, but may also provide prolonged service life. However, a major concern with most of the applications is that metallic components have achieved the limits of their performance capabilities. This shows that designers may find non-metallic material alternatives, specifically ceramic materials, for severe service applications.
Major Characteristics of Severe Service Components
The typical parameters required for operating the components in severe service conditions include thermal shock resistance, corrosion resistance, resistance to fatigue, hardness, strength and toughness.
Toughness is the key parameter as components that are less tough will probably fail catastrophically. Toughness in ceramic materials is defined as the resistance to crack propagation. In some cases, it can be measured using indentation methods, resulting in an artificially high value. The use of a single-edge notched beam can provide an accurate measure.
Strength is associated with toughness, yet refers to the single point at which catastrophic failure of the material occurs with the application of stress. It is typically called the ‘modulus of rupture’ measured by three or four-point bend strength measurements on test bars. The three-point test provides 1% higher value than the four-point test.
Although hardness can be measured with a number of scales including Rockwell and Vickers, the Vickers micro-hardness scale is well-suited for advanced ceramic materials. Hardness varies in proportion to a material’s wear resistance.
In valves operating in a cyclical method, fatigue is a main concern due to continuous opening and closing of the valves. Fatigue is a threshold of strength beyond which the material tends to fail below its normal bend strength.
Corrosion resistance is based on the operating environment and the media containing the material. A number of advanced ceramic materials are favorably compared with metals in this area, with the exception of the ‘hydrothermal degradation,’ which some zirconia-based materials undergo upon exposure to high- temperature steam.
The component geometry, thermal expansion coefficient, thermal conductivity, toughness and strength are affected by thermal shock. It is an area, which facilitates high thermal conductivity and toughness, and as a result, the metal components function effectively. However, advancements in ceramic materials are now providing acceptable levels of thermal shock resistance.
Available Options in Advanced Ceramics
Advanced ceramics have been in use for a number of years, and have gained popularity among reliability engineers, plant engineers and valve designers demanding high performance and value. Based on the specific application requirements, there are different individual formulations for a wide range of industries. However, four advanced ceramics are significant in the field of severe service valves, these include silicon carbide (SiC), silicon nitride (Si3N4), alumina, and zirconia. The materials for valves and valves balls are chosen based on the specific application requirements.
Two major forms of zirconia, both with identical thermal expansion coefficient and stiffness to that of steel, are used in valves. Magnesia partially stabilized zirconia (Mg-PSZ) have the highest thermal shock resistance and toughness, while the yttria tetragonal zirconia polycrystal (Y-TZP) is harder and stronger, but is vulnerable to hydrothermal degradation.
Silicon nitride (Si3N4) is available in different formulations. The gas pressure sintered silicon nitride (GPPSN) is the most common material used for valves and valve components, and provides high hardness and strength, exceptional thermal shock resistance and thermal stability, besides having average toughness. In addition, Si3N4 provides a suitable alternative to zirconia in elevated steam environments, preventing hydrothermal degradation.
With tighter budgets, specifiers can select from SiC or alumina. Both the materials have high hardness but are not tougher than zirconia or silicon nitride. This suggests that the materials are well-suited for static component applications, such as valve liners and seats, instead of valve balls or discs under higher stresses.
Advanced ceramic materials provide lower toughness and similar strength when compared to the metallic materials used in severe service valve applications, which include chrome iron (CrFe), tungsten carbide, Hastalloy and Stellite.
Use of Ceramics in Severe Service Valves
Severe service applications involve the use of rotary valves, such as butterfly valves, trunnion, floating ball valves and sprung. In such applications, Si3N4 as well as zirconia exhibits thermal shock resistance, toughness and strength to suit the most demanding environments. Due to hardness and corrosion resistance of the material, the service life of the component is increased by several times than that of metallic components. Other benefits include the performance characteristics of valve over its service life, especially in regions of maintaining shutoff capability and control.
This was illustrated in an application where a 65mm (2.6 inches) valve kynar/ RTFE ball and lining was exposed to 98% sulphuric acid plus ilmenite which was being converted into titanium oxide pigment. The corrosiveness of the medium meant service life for these components can last for up to six weeks. However, using a ball valve trim fabricated from Nilcra™ (Figure 1), which is a proprietary magnesia partially stabilized zirconia (Mg-PSZ) with excellent hardness and corrosion resistance, has provided uninterrupted service for three years without any detectable wear.
Figure 1. Ball valve made from Nilcra™
In linear valves, including angle, choke or globe valves, zirconia and silicon nitride are suitable for both plugs and seats, thanks to the ‘hard seated’ nature of these products. Similarly, alumina can be used for some liners and cages. Through match- lapping balls on seats, it is possible to achieve a high degree of sealing.
For valve liners, including spool, inlet and outlet or valve body liners, any of the four main ceramic materials can be used based on the requirements of application. The high hardness and corrosion resistance of the materials prove advantageous with respect to performance and service life of products.
Consider an example of a DN150 butterfly valve employed in a bauxite refinery in Australia. The high silica content of the medium provides high levels of abrasion to the valve liners. The liner and discs originally used was made of 28% CrFe alloy that lasted for only eight to ten weeks. However, with the introduction of a valve made of Nilcra™ zirconia (Figure 2), the service life was increased to 70 weeks.
Figure 2. Nilcra™ zirconia valve
Ceramics function well in most of the valve applications, thanks to their toughness and strength. However, it is their hardness and corrosion resistance, which help in improving the service life of valves. This, in turn, lowers the whole life costs through reduced downtime for replacement parts, lower working capital and inventory, minimal manual handling, and increased safety through less leakages.
Future of Ceramic Materials in Valves
The application of ceramic materials in high-pressure valves is one of the major concerns for a long period of time as these valves are under high axial or torsional loads. However, major players in the sector are now developing valve ball designs, which improve actuation torque survivability.
Another major constraint is size. The size of the largest valve seat and the largest valve ball (Figure 3) producible from magnesia partially stabilized zirconia is DN500 and DN250, respectively. However, most of the specifiers currently prefer ceramics for these components up to these sizes.
Figure 3. Large Nilcra™ zirconia ball valve
Although it is now proven that ceramic materials serve as an appropriate choice, a few simple guidelines needs to be followed to maximize their performance. The ceramic material should first be used only when there is a requirement to minimize the costs. Sharp corners as well as concentrations of stress should be avoided, both internally and externally.
Any potential thermal expansion mismatch must be taken into account at the design stage. In order to reduce the hoop stresses, it is necessary to hold the ceramics on the outside, not inside. Finally, geometric tolerances and the need for surface finishing should be carefully considered as these can add significantly and unnecessarily to cost.
It is possible to achieve an ideal solution for each severe service application by following these guidelines as well as the best practice in selecting materials and coordinating with suppliers from the start of the project.
This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials.
For more information on this source please visit Morgan Advanced Materials.